[Lustre-discuss] xserve raid

[Andreas Dilger]

On Oct 18, 2007  13:02 -0400, Brock Palen wrote:
> On Oct 18, 2007, at 4:42 AM, Andreas Dilger wrote:
> >For RAID 5/6 devices we recommend NOT having a partition table.   
> >The reason
> >is that the partition table offsets the data partitions by a small  
> >amount
> >(512 bytes usually) and this causes writes to span multiple RAID  
> >chunks and
> >unnecessary read-modify-write activity.
> >
> >For best performance, pick a RAID chunk size that divides evenly into
> >1MB (e.g. 4 or 8 data disks + parity).  The ldiskfs mballoc code works
> >to align the allocation with the RAID chunk size for best performance.
> 
> Thanks I will keep this in mind.
> 
> I did some basics test,  1MDS 1OST 1raid5 (half a xserve raid)  Using  
> tiobench on 1 client,  using no partition table netted about 5MB/s  
> faster for streaming read/write.  I will scale up my tests though and  
> try some other raid configurations.  Thanks for the help.

The other important note - don't use RAID5 for the MDS if at all possible.
It generates largely small, random 4kB IO and is much better served by
RAID1 or RAID1+0.

Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.